A metal line comprises a lattice of metal ions and non-localized free electrons. The metal ions are formed from metal atoms that donate some of their electrons to a common conduction band of the lattice, and the non-localized free electrons move with relatively small resistance within the lattice under an electric field. Normal metal lines, excluding superconducting materials at or below a superconducting temperature, have finite conductivity, which is caused by interaction of electrons with crystalline imperfections and phonons which are thermally induced lattice vibrations.
When electrical current flows in the metal line, the metal ions are subjected to an electrostatic force due to the charge of the metal ion and the electric field to which the metal ion is exposed to. Further, as electrons scatter off the lattice during conduction of electrical current, the electrons transfer momentum to the metal ions in the lattice of the conductor material. The direction of the electrostatic force is in the direction of the electric field, i.e., in the direction of the current, and the direction of the force due to the momentum transfer of the electrons is in the direction of the flow of the electrons, i.e., in the opposite direction of the current. However, the force due to the momentum transfer of the electrons is generally greater than the electrostatic force. Thus, metal ions are subjected to a net force in the opposite direction of the current, or in the direction of the flow of the electrons.
Scaling of semiconductor devices in advanced semiconductor devices is accompanied by corresponding scaling of metal interconnect structures. Because the size of copper grains is limited by the dimensions of metal interconnect structures, scaling decreases the size of grains in copper structures, while increasing the defect density. High defect density (i.e., smaller grain size of the metal) or high temperature typically increases electron scattering, and consequently, the amount of momentum transfer from the electrons to the conductor material. Such momentum transfer, if performed sufficiently cumulatively, may cause the metal ions to dislodge from the lattice and move physically. The mass transport caused by the electrical current, or the movement of the conductive material due to electrical current, is termed electromigration in the art.
In applications where high direct current densities are used, such as in metal interconnects of semiconductor devices, electromigration causes a void in a metal line or in a metal via. Such a void results in a locally increased resistance in the metal interconnect, or even an outright circuit “open.” In this case, the metal line or the metal via no longer provides a conductive path in the metal interconnect. Formation of voids in the metal line or the metal via can thus result in a product failure in semiconductor devices.
The electromigration failure is accelerated where copper is oxidized by diffusion of impurities from surrounding dielectric materials. Therefore, electromigration performance and reliability of metal interconnects can be enhanced by retarding oxidation of copper.